**Question:** Which among the following is NOT TRUE about H. influenzae?

A. Haemophilus influenzae is a Gram-negative, facultatively anaerobic, rod-shaped bacterium.
B. It is commonly found in the respiratory tracts of healthy individuals.
C. H. influenzae is responsible for a wide range of infections, including pneumonia, meningitis, and otitis media.
D. The majority of H. influenzae strains are sensitive to penicillin.

**Correct Answer: D. The majority of H. influenzae strains are sensitive to penicillin.**

**Core Concept:** Haemophilus influenzae is a bacterium that can cause various infections due to its ability to colonize the respiratory tracts of healthy individuals and its ability to cause disease. Penicillin is a beta-lactam antibiotic that exerts its antimicrobial effect by inhibiting bacterial cell wall synthesis, specifically by inhibiting the enzyme beta-lactamase and binding to penicillin-binding proteins (PBPs).

**Why the Correct Answer is Right:** The majority of H. influenzae strains are indeed sensitive to penicillin, which means they are susceptible to the antibiotic's antimicrobial effect and can be effectively treated with penicillin therapy. This is due to the bacterium's inability to produce beta-lactamase, an enzyme that degrades penicillin and renders it ineffective. Penicillin's binding to penicillin-binding proteins (PBPs) is essential for its antimicrobial activity.

**Why Each Wrong Option is Incorrect:**

A. While H. influenzae is a Gram-negative bacterium, the statement "Gram-negative" is incorrect for option D. This option inaccurately describes the bacterium as Gram-positive, which is incorrect.

B. The presence of H. influenzae in the respiratory tracts of healthy individuals does not imply that the majority of strains are resistant to penicillin. In fact, the majority of strains are sensitive to penicillin.

C. Although H. influenzae can cause various infections, the statement that it "is responsible for a wide range of infections" is not as specific as the correct answer about its susceptibility to penicillin.

**Clinical Pearl:** Penicillin remains a crucial antibiotic option for treating H. influenzae infections due to its ability to target the bacterium's cell wall synthesis and the bacterium's inability to produce beta-lactamase enzymes. This ensures that penicillin exerts its antimicrobial effect effectively against H. influenzae strains.